The present invention relates to digital-to-analog converters and specifically to such converters for use in addressing a digital memory.
Recently, several image display devices have been proposed utilizing a number of electron beams which scan different sections of the image display screen. One such device is disclosed in U.S. Pat. No. 4,028,582 entitled "Guided Beam Flat Display Device". In these devices, brightness variations may exist from one portion of the screen to another due to non-uniformity in the beam intensities for each of the screen sections. To solve this non-uniformity problem, a feedback system has been proposed wherein the electron beam intensity for each portion of the screen in periodically sensed and equalized to a given norm. U.S. Patent Application Ser. No. 749,178, now U.S. Pat. No. 4,126,814 filed on Dec. 9, 1976 by F. J. Marlowe and entitled "Electron Gun Control System," presents a system for providing this electron beam sensing and feedback control. This beam equalizing circuit has a digital random access memory in which is stored data representative of a mapping of the "grid voltage" vs. "beam current characteristics" of the particular gun. A previously digitized video signal addresses various storage locations in the memory containing the particular mapping data to yield the desired brightness level. The digital mapping data is converted to analog data and is used to bias the electron gun.
In these display devices, each of the electron beams simultaneously scan its portion of the display device screen. Therefore, the incoming serial video signal must be processed to provide a parallel feed to each of the simultaneously scanning beams. A converter for accomplishing this scanning is disclosed in U.S. Patent Application Ser. No. 740,770, now U.S. Pat. No. 4,080,630 filed on Nov. 10, 1976 by F. J. Marlowe and entitled "Line Scan Converter for an Image Display Device". This line scan converter as well as the gun control system use a digitized video signal. A well known method for digitizing an analog signal is the successive approximation technique. One version of this type of analog to digital (A/D) converter is described in Electronic Design, Volume 25, pages 55-57, Dec. 5, 1968.